Nigel Adams Conservative Keir Mather Labour A by-election was held in the United Kingdom parliamentary constituency of Selby and Ainsty in North Yorkshire on 20 July 2023, following the announcement on 10 June that the incumbent Conservative Party member of Parliament (MP), Nigel Adams, was standing down from Parliament with immediate effect.

He had been expected to receive a peerage as part of Boris Johnson's resignation honours, but had been removed from the final list by the House of Lords Appointments Commission (HOLAC).

The list of proposed peerages went to the commission, which rejected several names, presumed to include Dorries and Adams, as they were serving MPs who were not planning to stand down imminently.

[15] On 15 June the Labour Party announced 25-year-old Keir Mather, who most recently worked at the Confederation of British Industry,[15] as their candidate.

[18] On 22 June the Liberal Democrats announced their candidate as Matt Walker, who works as a National Health Service manager and serves as a councillor on North Yorkshire Council.

[27] On 8 July, Prime Minister Rishi Sunak campaigned with the Conservative candidate Claire Holmes in Selby and Ainsty.

Partners and commissioned by pressure group 38 Degrees, published on 6 July, found that Labour held a twelve-point lead over the Conservatives in the seat.

[29] Labour ultimately won the seat with a lead of 11.6 points over the Conservative candidate, a similar margin to that predicted by the poll.